During his service in World War I, Armour rose from a private to Staff Major in the Tank Corps. His conduct earned him an audience with George V. However, he lost his sight to a mustard gas explosion and surgeons had to add a metal plate to his head and left arm. During his convalescence, he regained the sight of his right eye, and began playing much more golf.

Armour won the French Amateur tournament in 1920. He moved to the United States and met Walter Hagen, who gave him a job as secretary of the Westchester-Biltmore Club.[citation needed] He became a U.S. citizen at this time. He competed in important amateur tournaments in the U.S. for a time, and in 1924 became a professional golfer.

Armour also won the Canadian Open three times, a feat exceeded only by Leo Diegel, who won four.

At the Shawnee Open in 1927, Armour scored the first ever "Archaeopteryx" (15 or more over par) when he made a 23 on a par 5, for 18-over par. This still stands as the highest score on one hole in PGA history. This historic performance happened just one week after winning the U.S. Open.

Armour co-wrote a book How to Play Your Best Golf All the Time (1953) with Herb Graffis. It became a best-seller and for many years was the biggest-selling book ever authored on golf. A series of 8mm films based on the book was released by Castle Films including Short Game parts I and II, Long Hitting Clubs, Grip and Stance.

Armour died in Larchmont, New York, and was cremated at the Ferncliff Cemetery in Hartsdale, New York, but is not interred there. Some modern golf equipment is still marketed in his name.

Armour is succeeded by his grandson, Tommy Armour III, who is a two-time winner on the PGA Tour and currently holds the record for the lowest total score on 72 holes (254), which he set in his second PGA Tour victory at the Valero Texas Open.

NYF = Tournament not yet founded
NT = No tournament
DNP = Did not play
WD = Withdrew
CUT = missed the half-way cut
R64, R32, R16, QF, SF = Round in which player lost in match play
"T" indicates a tie for a place
Green background for wins. Yellow background for top-10.

Sources: U.S. Open and U.S. Amateur,[4] Amateur Championship:1920,[5] 1921[6]